The long-term goal of this project is to understand the structure and function of CD33, a membrane glycoprotein expressed on normal myeloid progenitor cells and myeloid leukemia cells. Specifically, we propose to identify ligands for CD33 and analyze their expression during mouse embryogenesis. Previous phage display studies from our laboratory have identified a peptide motif not corresponding to any known human protein that binds to the extracellular domain of CD33. We propose to further characterize the interaction of this peptide with CD33, identify natural ligands for CD33 on cell lines and on mouse tissues by immunolocalization, analyze the expression of CD33 and its ligands concurrently during mouse development, and study the effects of CD33 on murine development by creating and analyzing a CD33 "knockout" mouse. In addition to improving our understanding of mammalian hematopoiesis, these studies will also facilitate CD33-mediated targeting of gene therapy vectors to hematopoietic cells for the treatment of myeloid leukemia and other disorders of myeloid cells.